1. Technical Field
The present invention relates to a lubricant for fluid dynamic bearing, a spindle motor equipped with a fluid dynamic bearing with the lubricant used therein, and a magnetic disk device equipped with the spindle motor.
2. Background Art
Fluid dynamic bearing have been used as a bearing for disk-driving spindle motor in information-recording device such as magnetic disk device (hard disk drive) In the recent growing demand for miniaturization of hard disk drive, there is a need for miniaturization of motor such as a spindle motor for driving magnetic disk. Shipment of small-sized hard disk drive is growing drastically, especially in the application for installation in potable small-sized audio recording/reproducing system, and will be so in applications such as in-car system and cellphone.
Small-sized hard disk drive in such applications are turned on and off more frequently than common hard disk drive installed in personal computer, and the motor therein repeats start up and shut down accordingly. In the fluid dynamic bearing installed in such a motor, the shaft therein occasionally becomes in contact with the metal supporting parts such as sleeve when the motor is turned on or off. It is thus inevitable that the shaft and the sleeve are worn away. It is necessary to reduce the wear loss as much as possible, to obtain a hard disk drive that satisfies requirements in performance such as rotational accuracy stable for an extended period of time.
An example of the conventional means for preventing wear of metal parts constituting the fluid dynamic bearing is a method of adding a phosphorus compound such as a phosphoric ester to the lubricant filled in fluid dynamic bearing (see e.g., Japanese Patent Application Laid-Open No. 2001-240885). The phosphorus compound forms an extreme-pressure film on metal surface, preventing the contact and reducing the wear between the metal parts. The extreme-pressure film is a film relatively softer than the metal that is formed in the reaction caused by frictional heat of an extreme-pressure agent such as a phosphorus compound with the newly-generated metal surface activated by wear.
Although there is a need for miniaturization of the fluid dynamic bearing along with miniaturization of motor, the amount of the lubricant used in bearing is smaller in small-sized fluid dynamic bearing. Some of the lubricant vaporizes in the fluid dynamic bearing during use at high temperature, and thus, decrease in the amount of the lubricant by vaporization exerts a significant influence on lifetime of the fluid dynamic bearing when the amount of the lubricant is smaller.
Vaporization of the lubricant seems to occur in the following way.
Some of the bonds in the molecular structure of organic compound commonly used as a base oil of the lubricant are cleaved by heat and oxidation, generating low-molecular weight compound. The compound vaporize easily because they have higher vapor pressures. In addition, the reaction generating the low-molecular weight compound is known to be accelerated by metal catalysis.
Small-sized hard disk drive, which are commonly operated at high rotational speed and becoming used more in in-car device, are used inevitably at a temperature higher than conventional disk drive. Portable device are often used outdoor under direct sun, and thus, may be operated at high temperature during use. For those reasons, it is quite important to improve heat resistance of the lubricant.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2001-240885
[Patent Document 2] Japanese Patent Application Laid-Open No. 2002-348586
[Patent Document 3] Japanese Patent Application Laid-Open No. 2003-221588
[Patent Document 4] Japanese Patent Application Laid-Open No. 2004-155873